Conventionally, speed control of a motor is accomplished by feeding back the current rotation angle .theta. of the motor for comparison with the rotation angle .theta.ref of the command value and multiplying the difference (.theta.ref-.theta.) by a suitable gain so as to vary the voltage for driving the motor. FIG. 1 is a block diagram of a conventional motor, wherein K1, K2 are gains, Tm=RJ/(RD+Kt.multidot.Ke), and Km=Kt/(RD+Kt.multidot.Ke), where R(.OMEGA.) is the resistance of the armature, J(Kgcm.sup.2) is the inertia of the armature, Kt(Kgcm/A) is a torque constant, and Ke is a counter electromotive force constant. In a motor of this type each of the gains K1, K2 of the P-type controller is designated based on a comparison of the coefficients of the characteristic equation of the transfer function from the command value .theta.ref to the current value .theta. with those of a desired transfer function defined based on the pole selected for the desired response. The response time is varied in accordance with the gains K1, K2.
In this conventional servo motor, however, it is necessary to execute various calculations, especially multiplications, after the feedback of the current value .theta. and the calculation of the deviation from the command value .theta.ref. Accordingly, the system requires a control circuit of relatively large scale and size for executing the complex calculations. This makes the system large and costly.